1. Field of the Disclosure
The present disclosure relates generally to scanners, and more specifically, to a reduced height illumination assembly for a scanner.
2. Description of the Related Art
High speed scanners typically require high intensity illumination. One example of a light source that provides the required illumination is an external electrode xenon fluorescent lamp. However, various problems are associated with the use of a xenon fluorescent lamp as a light source. For example, a high power xenon fluorescent lamp must be used to generate the required illumination. Such high power lamps may generate excessive heat. To address this issue, a cooling fan and a vent may be positioned on the scanner; however, this adds cost and acoustic noise and, in some cases, may result in potential contamination through the vent. Further, the high power xenon fluorescent lamp needs a high frequency and high voltage inverter, which further adversely impacts the scanner both in terms of safety and signal quality.
Recent technological developments in the field of scanners have provided another light source, white LEDs (light emitting diodes). In many cases, one high power LED may be used in conjunction with a light guide to generate the required illumination. Alternatively, an array of medium power LEDs may be used. The array of medium power LEDs generates more light and is desired for high speed scanners. However, it is desired to use as few LEDs as possible to reduce the cost of the device. The white LEDs may be generally blue LEDs that use a blue LED die with yellow phosphor to form white light. Light produced by such LEDs may not be sufficiently diffused or mixed and direct incidences of such light at a target area, such as a sheet to be scanned, is not desired.
Using an array of LEDs to provide illumination for document scanner has become popular. There are various ways to arrange the LEDs, collect light and direct them to the intended scan area. Each configuration usually balances uniformity, the degree of diffusion and efficiency and is achieved via optical simulation and empirical processes. Adequate intensity is needed to provide sufficient signal to noise ratio for the intended scan speed. Uniformity is needed to preserve the consistency of the noise to signal ratio across the page, and also adequate stability in the width direction of the illuminated area. Diffusion is needed to avoid dependence of scanned image to the reflective properties of the document surface. Under perfect diffuse illumination, the calibration strip would provide ideal calibration to the scan to produce scanned image free of artifacts of the illumination from the scanner. For example, if the calibration strip is of a matte property, it will reflect light in one way. If the calibration strip is glossy, the specular reflection may produce a mirrored image that overlaps on top of the document image. Such a defect may appear as a banding pattern consistent with the LED array pattern. Another common defect is the deviation of neutral tones from being truly neutral, which is important to adequate color quality. With diffuse illumination, a scanner tuned to one type of paper for an ideal grey tone, will render similar grey tones for other common media types. With imperfect diffuse illumination, different media types may show different color hues that deviate from neutral, which can be highly objectionable. Therefore when intensity and efficiency is not an issue, a more diffuse illumination is highly desired.
C-shaped illumination assemblies having a primary C-shaped reflector positioned directly in the light ray path of a LED array or light source that is aimed directly into the bight portion or central portion of the C along with a secondary reflector positioned opposite the open end of the C have been used to achieve a more diffuse illumination of a target area of a scanner. However such an arrangement increases the overall height of the primary reflector because the radius of curvature of the primary reflector is increased so that the light rays reflected by the primary reflector are not blocked by the LED array or light source and its mounting structure. Having a more compact illumination assembly would help to reduce the overall height of the scanner.
Another important aspect of when using LEDs or LED arrays is thermal management. Both light output and life of a LED die is dependent on its operating junction temperature. The LED die is the internal semiconductor junction of the LED. Maintaining the LED array light output stability during a scan job is important to maintaining the consistency of scan image quality. The LED array must also perform consistently over the life of the scanner which also requires that the LED die temperatures of the LEDs in the array do not exceed their rated temperature.
Because of the potential excessive heat generated during a scan from the illumination assembly, particularly when a number of sheets of media are being scanned via an automatic document feeder, the LED array is usually mounted onto a heatsink or its equivalent to keep the LED dies to a desirable temperature range. The heatsink, or its equivalent, perform best when its mass and surfaces are large and when it is ventilated well such as when it is exposed to open air. The C-shaped illumination assemblies of the prior art have the LED array printed circuit board attached to a heatsink that is positioned between the C-shaped reflector and the main optical frame of the scan head. Because the C-shaped reflector is easier and more cost effective to manufacture by plastic injection molding, the C-shaped reflector acts as a cover to hinder airflow to the heatsink and therefore decrease the efficiency of the heatsink. Although the C-shaped reflector can be made of metal, if cost is not an issue, doing so will increase the overall weight of the scan head assembly due to its substantially large volume which is a consideration in scanning systems where the scan head is moved during scanning.
Accordingly, it will be appreciated that a compact, efficiently heatsinked, and lower weight illumination assembly for a scanner that provides diffuse high intensity illumination in an efficient manner is desired.